Influence of steric interactions and random side chain variations on the mesomorphic properties of bowlic mesogens

The effect of structural variations on the mesomorphic nature of columnar liquid crystals of general structure I is studied. X-ray crystal structure analyses of compounds la and 2a reveal the columnar organization of the macrocyclic cores in the solid state. The up and down asymmetry of the cores is resolved in an alternating sequence of up-up and down-down intermolecular contacts. Specific interactions between the R substituents are present, which influence the stacking distance between the cores in the down-down intermolecular contacts. Statistical incorporation of different side chains on the same macrocyclic core produces, when the difference in length between the alkyl chains is appropriate, random mixtures having wide mesomorphic ranges. The increase of disorder associated with the statistical side chain distribution on the core and with the structural heterogeneity of each component of the mixture have the effect of depressing mainly the crystal-mesophase transition temperature.